Many mothers in the U.S. start infants on solid foods -- including peanut butter, meat, and french fries -- earlier than experts recommend, and half of them do so with their doctor's support, according to new research from the U.S. Centers for Disease Control and Prevention.

The study found that 40.4 percent of U.S. mothers interviewed from 2005 to 2007 said they introduced solid foods to infants before they were 4 months old -- that represents an increase of about 29 percent from earlier studies, the researchers reported today in the journal Pediatrics.

More than half of the mothers (55 percent) cited a doctor's advice as one of the reasons for introducing solids before 4 months.

"With multiple sources of information on infant feeding and care from healthcare providers, family, friends, and media, specific information on the timing of solid food introduction may be conflicting and not necessarily sensitive to the needs of mothers," the authors said.

Among mothers who introduced solid foods earlier than 4 months, the mean age of the children at introduction was 11.8 weeks, and 9.1 percent of early introducers gave solids to infants younger than 4 weeks, they added.

The authors noted that if they factored in the American Academy of Pediatrics' (AAP) 2012 feeding recommendation to avoid giving solid foods until 6 months, 92.9 percent of their analytic sample would have been "early introducers."

Solid foods included dairy {other than milk) such as yogurt, soy foods (other than soy milk) such as tofu, infant cereals and starches, fruits and vegetables, french fries, meat and chicken, fish, peanut butter or nuts and sweets.

Introducing solids early may increase the risk of some chronic diseases, the authors noted, including diabetes, obesity, eczema, and celiac disease.

The study used monthly 7-day food-frequency questionnaires throughout infancy to pinpoint infant age when solid food was introduced, and to classify whether the child was breastfed only, given only formula, or both at the time of solid-food incorporation.

Among early introducers, 52.7 percent exclusively formula-fed their infants; 50.2 percent mixed formula with breastfeeding, and 24.3 percent only breastfed.

Younger, unmarried, less educated, and Women Infants and Children (WIC) recipients were more likely to introduce solids, the authors said. Women who had delivered more than one child comprised 70.9 percent of early introducers. Only 29.1 percent of early introducers were first-time mothers.

The AAP recommends babies be exclusively breastfed for their first 6 months and that breastfeeding be maintained throughout the first year.

Solids Too Soon: Doctors to Blame?

Healthcare providers might be as equally confused about infant feeding guidelines as mothers, the authors wrote, saying some clinicians "may rely on their own infant feeding experience rather than evidence-based guidelines when counseling women."

The authors cited 40 percent of U.S. obstetricians from an earlier study who rated their infant-feeding training as "inadequate." Pediatricians and pediatric residents also have reported similar sentiments.

"Given the dearth of evidence for reason for solid food introduction by milk feeding type, it is not well understood why mothers of formula-fed infants were more likely than mothers of breastfed infants to cite a healthcare professional's advice as a reason for introducing solid foods earlier than recommended by the AAP," the authors said.

How mothers perceive their doctor's advice may indicate differences in clinician training, opinion, or counseling strategy, specific to formula-fed infants, the study said.

"It is possible that clinicians recommend earlier solid food introduction for formula-fed infants simply because they think that the solid food recommendation is specifically for breastfed infants, because much of the attention on infant feeding is focused on the goal of exclusive breastfeeding," the authors wrote.

New mothers self-reported their feeding preference when they discharged from the hospital.

The authors said theirs was the largest prospective study on U.S. babies and their feeding habits, but the study had limitations, most notably that the sample was not nationally representative, skewing to predominantly mothers who were white, middle income, and English speakers.

They noted that mothers in lower socioeconomic households were more likely to introduce solids earlier, the authors wrote. For 72 percent of Latina mothers in New York City receiving WIC benefits, baby crying was associated with a sign of hunger.

"It is reasonable to think that our sample may underestimate the prevalence of early solid food introduction," they said.